The author of the letter to the Hebrews is obviously concerned with the order of the high priesthood. I can't help but read in it a controversy brewing within the community as to how priests were to act now that the one High Priest had lived, died, and rose again.
"He is able to deal gently with the ignorant and wayward, since he himself is subject to weakness; and because of this he must offer sacrifice for his own sins as well as for those of the people. And one does not presume to take this honor, but takes it only when called by God, just as Aaron was."
Do you maybe hear discernment process in there? Do you hear maybe holier than thou priests running amuck?
Anyway, where the rubber meets the road is in Mark's gospel as James and John, the sons of Thunder, come to Jesus hoping for the number 1 and 2 spots in the club. They were part of the elite 3 who got to see Jesus Transfigured, and are no doubt thinking they've got Secretary of State and Chief of Staff in mind. Just as those who have SoS and CoS in mind, they might just be too dumb to know what that position means.
The other 10 hear of what James and John are up to, and, not surprisingly, get annoyed. And so, Jesus has to once again remind them - rule of the road #2 - there will be no competition - by giving them his own mission statement. Jesus, the Son of God, the #1 of #1s came not to be served but to serve.
That's it. No competition. No High Priests run amuck. Just service to God and to one another in humility. The kingdom of God is like that.